The present invention relates to a flux for soldering and a solder composition comprising the same and a soldering method using the same, and more particularly to a flux for soldering which does not necessitate cleaning after soldering electronic parts on a substrate and a solder composition comprising the flux for soldering and a soldering method using the solder composition.
Conventionally, after electronic parts are soldered on a substrate, the substrate should be cleaned up with a solvent containing Flon 113 (CFC-113) as a main component in order to mainly remove a flux. However, the use of a specific Flon represented by Flon 113 is undesirable for reasons of environment problems in recent years. Therefore, it is urgently required to develop a method for cleaning a substrate with a cleaning agent other than Flon after soldering or a soldering method in which a step of cleaning is omitted.
Among the above-mentioned methods, as reported in "MITSUBISHI DENKI GIHO" Vol 67, No. 6, p. 532-536 (1993), is a soldering method in which a step of cleaning is omitted, since ionic components such as a halogen ion of a halogenous activator, which is used for removing a layer of oxide, are contained in a flux or a creamy solder used for soldering and remain after soldering. Accordingly, when the electronic parts-soldered printed wiring board is used for a long period of time, the remaining ionic components accelerate the generation of ion-migration of an electrically conductive metal and thereby there is a danger for occurrence of poor insulation in a wiring of the substrate. Moreover, since corrosion of a wired metal is apt to occur, danger of breakage or snapping of the electrical conductive metal is incurred. From the reasons as mentioned above, when soldering is carried out by using a halogenous activator having an excellent soldering property without cleaning after soldering, danger for occurrence of electrical failure caused by a remaining halogen ion on the substrate is increased. Accordingly, as a material for soldering which does not necessitate cleaning after soldering, there are proposed a flux and a creamy solder in which the halogenous activator is not used or the amount of a halogen atom is reduced. Also, as a soldering method which does not necessitate cleaning after soldering, there are proposed a soldering method using the above-mentioned flux or the above-mentioned creamy solder. When the flux or the creamy solder is used, or the above soldering method is employed, electrical reliability (hereinafter also referred to as metal ion-migration resistance) is insured.
As a flux not containing a halogenous activator, for instance, there is proposed a flux generally referred to as a low-solid flux in which an organic acid such as an aliphatic carboxylic acid not containing halogen components is used as an activator and the total amount of remaining solid matter of flux, that is, flux components other than solvent is decreased as described in Japanese Unexamined Patent Publication No. 262893/1992.
Also, for instance, as described in Japanese Unexamined Patent Publication No. 86390/1991, there is proposed a creamy solder in which a halogenous activator is not used or the amount of a halogen atom in the activator is decreased by using a sulfonic acid salt as an activator.
Furthermore, for instance, as described in Japanese Unexamined Patent Publication No. 269896/1992, there is proposed a soldering method in which a flux is not used or a flux showing low activity such as a low-solid flux is used, and soldering is carried out in an atmosphere of inert gas such as nitrogen gas.
However, when the above-mentioned low-solid flux in which an organic acid such as an aliphatic carboxylic acid not containing halogen components is used as an activator or the creamy solder in which an activator containing a halogen atom is not used or the amount of a halogen atom is decreased by using a sulfonic acid salt as an activator is used, as compared with the case when a flux in which a halogenous activator is used or a creamy solder in which a halogenous activator is used is employed, metal ion-migration resistance and corrosion resistance are improved. However, the ability for removing the layer of metal oxide is small since the halogenous activator is not used and the resin film is thin. Accordingly, there are some fatal defects that ability for restricting the oxidation of the surface of metal at high temperatures is lowered and soldering property becomes worse. Therefore, there are problems that soldering delinquency is easily generated and productivity becomes worse.
Also, according to the soldering method in which no flux or a flux showing low activity is used and soldering is carried out in an atmosphere of inert gas such as nitrogen gas, there is a fatal defect in that running costs become great, and there is a problem in that simplification of the steps for soldering and lowering of costs which are the merits of this method cannot be accomplished.
An object of the present invention is to provide a material for soldering showing: excellent electric reliability in comparison with a conventional soldering method which does not necessitate cleaning after soldering, and excellent soldering properties improved processability of soldering without cleaning after soldering, and a soldering method using the material for soldering.
This and the other objects of the present invention will become apparent from the description hereinafter.